Some fibre reinforced composite components comprise an inner rigid foam core sandwiched between outer layers of fibre reinforced composite material. Foam cores are used extensively in the manufacture of fibre reinforced plastic parts to increase the rigidity of the finished article by separating two fibre-reinforced layers, acting as structural skins, with a low-density core material, acting as a structural core. The fibre-reinforced layers are bonded to the low-density core material by a layer of resin material. This construction is commonly called a sandwich panel in the composite industry.
The primary functions of a structural core are to increase panel rigidity, by reducing the overall deflection under load and onset of global panel buckling, and to prevent skin wrinkling and localised buckling.
It is often desired to maximise the mechanical properties of the foam for a given density to enable the lightest weight core to be selected to transfer the structural loads between the fibre reinforced layers. The foam must also be compatible with the materials and manufacturing process used to make structural composite skins.
There is a general need to reduce both construction cost and component weight of composite laminated articles. When a fibre reinforced layer is to be bonded to a core layer it is necessary to provide sufficient resin in the fibre reinforced layer to enable complete bonding to the core layer. There is a need in the art for foam cores that can be securely and reliably bonded to fibre reinforced layers over the interface therebetween that permits a minimum amount of resin to be required for such bonding, in order to minimise the weight and material cost for achieving a given structural performance providing particular mechanical properties.
Furthermore, the size of foam core pieces is limited by both the foam manufacturing process and the handleability of the foam pieces, in order for operators to be able to fit the foam into the mould being used to form the composite component. It is increasingly common for a foam core to be supplied pre-machined to speed up assembly. These foam kits can be made into a jigsaw of foam parts with self assembly features, such as dog bones or serrated edges, to speed up the assembly within the mould and to provide correct positioning of the core into a complex moulding. Depending on the complexity of the core, the machining can lead to considerable amounts of foam material being wasted.
There is a general need to reduce the amount of foam core material being wasted in the manufacture of composite laminated articles.
Low density structural foams (having a density of from 50-600 g/L) currently used in the composite industry that have the highest mechanical and thermal performance are cross-linked polyvinyl chloride (PVC) foam, styrene acrylonitrile (SAN) foam, and polymethacrylimide (PMI) foam. When the outer layers of fibre reinforced composite material are preset as pre-pregs, these foams are suitable for high temperature pre-preg processing at temperatures from 75-160° C., depending on the foam type, in which processing the foam should resist at least 1 bar vacuum pressure for extended periods of time during the pre-preg cure. Other such known foams can be used for lower temperature applications at processing temperatures of from 20-75° C., for example using resin infusion processing, which is known in the art for the manufacture of articles such as boat hulls, decks and bulkheads.
These known foams are made from batch processes and are both time consuming and expensive to produce. These foams have varying degrees of cross-linking making them more difficult to recycle as they cannot be re-melt processed, unlike a true 100% thermoplastic material.
Pre-expanded polystyrene (PS), known in the art as EPS, is commonly used to manufacture low density, low cost foam blocks and moulded parts. It has limited historical use as a structural core in the composite industry, because the polystyrene foam has a low heat resistance and low mechanical properties. Polystyrene cores have been used with epoxy room temperature curing laminating resins but are not suitable for use with polyester and vinyl ester resins, because the styrene used in the resin material attacks and dissolves the polystyrene foam.
The use of EPS for resin infusion and injection processes (VARTM) has not been successful because commercially available EPS grades are relatively porous and the foam absorbs large amounts of resin during the injection process. The resins designed for infusion processes are generally low in viscosity and may contain diluents. In addition, it has been found that some epoxy resins attack and soften EPS during the resin infusion (VARTM) processing. This is due to the combination of the exothermic heat of reaction from the curing epoxy resin, which raises the temperature of the foam, and the low chemical resistance and high porosity of the foam. Usually epoxy resins are selected for demanding applications and a higher performance core is usually preferred to minimise the final component weight.
It is known to add polyphenylene oxide (PPO), also known as polyphenylene ether (PPE), to polystyrene to provide a higher temperature-resistant material with higher mechanical properties. Unusually for thermoplastics, the PPO is miscible and compatible with polystyrene (PS). This compatibility gives the mixed PS/PPO a range of properties, generally the property being related to or proportional to the amount of the material present by a rule of mixtures average of the two polymer properties. The more expensive PPE (PPO) increases the glass transition temperature (Tg), strength and modulus of the blend. This is a key feature as in less compatible polymer blends the material would still show some softening at the temperature of the lowest thermal resistant component. This gives a cost effective higher temperature, tough thermoplastic.
PS/PPO is used for manufacturing some industrial and household plastic goods requiring higher heat resistance. The amount of PPO added is proportional to the improvement in Tg and mechanical properties. The compatibility of PS/PPO is known, and has been marketed commercially, for example by GE Plastics as Noryl®.
PPO/PS is currently commercially available as an unexpanded bead containing a residual amount of a blowing agent, in particular pentane, for producing low density foams (less than 100 g/L) via an expanded polystyrene (EPS) type process. The main use of PPO/PS has been in low density (less than 100 g/L) insulation applications where additional thermal resistance is required such as the first part of the thermal insulation on a hot-water boiler tank. PPO/PS is also used to manufacture high impact performance cycle helmets due to its higher mechanical properties.
EPS/PPO foams are niche products and not well known in the packaging and construction markets. More utilised and marketed are higher impact performance foams such as EPE (Expanded Polyethylene) and EPP (Expanded Polypropylene). These polymers are not ideal for use as a structural core for epoxy composite laminates as they are difficult to bond to, have low modulus and poor thermal resistance showing early softening and creep before their Tg. A useful characteristic of EPS/PPO blends is the retention of modulus close to its Tg value leading to little creep and softening deflection under load.
There is a general need to produce composite laminated articles comprising a foam core having high mechanical properties, and high thermal properties, that can be readily produced at low cost and using conventional composite manufacturing processes.